Airway smooth muscle tone can be altered by histamine, serotonin, prostaglandins, acetylcholine, catecholamines, and a host of other endogenous substances that are released into the pulmonary or bronchial circulations, the pulmonary interstitium, or the airway walls. Past studies on intact animals and isolated airways have attributed the bronchomotor effects of these substances either to a direct effect on airway smooth muscle cells or to activation of reflex pathways with subsequent neurotransmitter release. Virtually ignored in these reports is the possibility that several of these substances may profoundly alter bronchomotor tone by modulating neurotransmiter release via prejunctional receptors on parasympathetic and sympathetic nerve varicosities. That such presynaptic mechanisms exist has been demonstrated in a vast number of studies on the gut, heart, and blood vessels. The release of acetylcholine and norepinephrine from these tissues was markedly enhanced or depressed by several endogenous substances at concentrations lower than those required to stimulate smooth muscle cells directly. Indirect evidence from a few recent studies indicates that similar mechanisms may also modulate cholinergic and adrenergic neurotransmission in airways. Therefore, the primary objective of this proposal is to employ superfuusion techniques to provide direct evidence for presynaptic modulation of neurotransmitter release in isolated canine airways. The effects of various endogenous substances on spontaneous release during basal conditions and on release during nerve stimulation will be investigated. The release of endogenous acetylcholine will be measured by an assay recently developed in our laboratory. Norepinephrine release will be measured after intraneuronal stores are labelled with 3H-norepinephrine. Our findings may have important implications with respect to the normal regulation of bronchomotor tone as well as regulation during disease states when the local environment of nerve terminals may be substantially altered by the release of endogenous substances from surrounding cells.